This application is based on Korean Patent Application No. 2001-43505 filed Jul. 19, 2001, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates in general to an optical head, and more particularly to an optical head with a graded index (GRIN) lens.
2. Description of the Related Art
Optical pickups for recording data in or reading data from a high-density data storage medium, such as an optical disk, include a head for recording a predetermined data in a recording layer of the data storage medium or reading the data from the recording layer using a laser beam emitted from a laser diode.
The data recording density of a high-density data storage medium such as an optical disk is dependent on the spot size of a laser focused on the recording layer of the storage medium. The spot size depends upon the wavelength of a laser emitted from a laser diode for a header and the focusing power of an optical system including the header. The larger the data recording capacity, the shorter the wavelength of a laser beam for recording and the greater the focusing power of the optical lens system.
Increasing the recording density of a storage medium based on the above principle is limited by diffraction. Therefore, there is a need for a new optical head capable of increasing the recording density over the limitation by diffraction. As a result of the effort made to overcome the limitation by diffraction, a near-field optical head capable of ultra high-density data recording and reading based on evanescent coupling was developed recently.
FIG. 1 shows an optical system for a near-field optical head according to the prior art. In FIG. 1, reference numeral 2 denotes a disk and reference numeral 4 denotes a recording layer coated on the surface of the disk 2. An air-bearing flying slider 6 is shown adjacent to the recording layer 4. The flying slider 6 and the recording layer 4 may be spaced apart a distance that is less than the wavelength of a laser beam 13 for data recording. A solid immersion lens (SIL) 8 is mounted in the flying slider 6, coaxially with the flying slider 6, facing the recording layer 4 of the disk 2. The SIL 8 is a hemispherical or super-hemispherical lens having a flat surface that faces toward the recording layer 4 and a hemispherical or super-hemispherical surface that faces away from the recording layer 4. A magnetic field modulation coil 10 for correcting a head tilt is arranged around the SIL 8 in the flat surface of the flying slider 6 facing the recording layer 4. Also, an objective lens 12 is coaxially arranged above the flying slider 6.
The size of a laser spot condensed by the objective lens 12 markedly reduces while passing through the SIL 8. That is, the SIL 8 reduces the laser spot size to 1/n of the size before entering the SIL 8 (or to 1/n2 if the SIL 8 is a super-hemispherical lens), where n is the index of refraction of the SIL 8. This reduced laser spot size occurs on the bottom of the SIL 8, i.e., in the vicinity of the recording layer 4.
When the distance between the SIL 8 and the recording layer 4 is maintained to be less than the wavelength of the laser beam 13, the spot focused on the bottom of the SIL 8 is wholly transmitted onto the recording layer 4 through evanescent coupling, where the spot on the bottom of the SIL 8 and the spot on the recording layer 4 are the same in size, thereby enabling near-field recording.
The conventional near-field optical head shown in FIG. 1 increases the numerical aperture (NA) of the optical system by reducing the spot size of the laser beam 13 being focused onto the recording layer 4 using two lenses, i.e., the objective lens 12 and the SIL 8.
As the NA of an optical system is increased, the range of the dimensional tolerance of the optical elements constituting the optical system becomes narrow. Therefore, there is a need to accurately adjust the spacing between the SIL 8 and the recording layer 4 without tilting or decentering the objective lens 12 in order to reduce aberrations in the optical system having the optical head shown in FIG. 1, which increases the manufacturing costs of the optical head.
The conventional optical head shown in FIG. 1, which uses both the objective lens 12 and the SIL 8, also has a problem of access speed reduction due to its increased volume and weight.
FIG. 2 shows the structure of an optical system of another optical head configured to solve the problems of the optical head shown in FIG. 1. The optical head shown in FIG. 2 uses a graded index (GRIN) lens. For convenience, a semiconductor laser diode, which is disposed on the left side of the GRIN lens 14 in FIG. 2, for emitting a laser beam for recording is not illustrated; only radiation of a laser beam from the semiconductor laser diode is illustrated. In FIG. 2, reference numeral 8 denotes an SIL.
The optical system of the conventional optical head shown in FIG. 2 using the GRIN lens 14 and the SIL 8 can reduce aberrations in the optical system and can increase the NA with a simple configuration. However, this optical system still needs a coaxial alignment between the GRIN lens 14 and the SIL 8, and it is difficult to implement the above described optical system in a miniature, light-weight optical head.
FIG. 3 shows an optical system of another conventional optical head using a GRIN lens. A metal layer 16 is attached to an surface of the GRIN lens 14, facing a recording layer (not shown). A hole 18 is formed in the metal layer 16 to expose a portion of the surface of the GRIN lens 14. The metal layer 16 limits laser emission through the GRIN lens 14. In FIG. 3, reference numeral 20 denotes a semiconductor laser diode for emitting a laser beam towards the GRIN lens 14 through an objective lens 12.
The conventional optical head shown in FIG. 3, which uses the metal layer 16 instead of an SIL, has structure that is simpler than that of the optical head shown in FIG. 2. However, it is difficult to form the hole 18 at the center of the metal layer 16 to be aligned with the optical axis. For example, it is very difficult to form the metal layer 16 on the surface of the GRIN lens 14 having a 200-μm diameter, and to form the hole 18 having a 100-nm diameter at the center of the metal layer 16.
Although a slit of a desired size can be formed by partially melting and vaporizing the metal layer 16, suitable materials therefor, which have low reflectance, high absorbance, and low melting point, are extremely limited. In addition, when the slit is smaller than the wavelength of a laser beam passing through the same, light transmitting efficiency, expressed as a ratio of the output beam intensity to the input beam intensity, is lowered. For example, in a near-field probe head using an optical fiber, the light transmitting efficiency is about 10−4-10−7 for a slit of 100 nm. In this aspect, the optical head shown in FIG. 3 is unsuitable for practical use in the field due to its poor optical efficiency.